1. Field of the Invention
The present invention relates to the field of reproducing images in large numbers, particularly image production by printing technology, and more particularly to the field of negative-acting photolithographic printing plates and methods of their use.
2. Background of the Art
The art of lithographic printing is based on the immiscibility of oil and water and the relative oleophilic and oleophobic properties of materials that undergo chemical changes, particularly polymerization and depolymerization. In one lithographic printing format, the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water while the image area accepts ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. In the transfer printing process, commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is reproduced.
A generally used type of lithographic printing form precursor has a light sensitive coating applied to a substrate. In most commercial environments an aluminum base support is used. In flexographic printing, a polymeric substrate, such as polyester film is used. Negative working lithographic printing form precursors have a radiation sensitive coating which, when image-wise exposed to light, hardens in the exposed areas. On development the non-exposed areas of the coated composition are removed leaving the image. Positive working lithographic printing form precursors have a radiation sensitive coating which, when image-wise exposed to light, decomposes and/or loses oleophilic properties in the exposed areas. On development the exposed areas of the coated composition are removed leaving the image.
Polyvinyl acetals polymers, especially polyvinyl butyrals are used in a very wide range of products as binders and oleophilic enhancers, primarily due to their excellent film forming, outstanding mechanical characteristics and their ability to contribute to the oleophilic properties of the imaged lithographic layer. These polymers are also known as materials with very good resistance to chemical attack. The polyvinyl butyrals or polyvinyl formals belong to the classes of materials that are not soluble in aqueous developers used in the preparation of printing plates or printing circuit boards. Many different polymers have been proposed for use as binders in conventional analogue negative-working UV-sensitive compositions that provide the required aqueous solubility. Particular advantages have been achieved for polymers containing hydroxyl groups or carboxyl groups in the acetal moieties. These binder polymers are described, by way of non-limiting examples, in U.S. Pat. Nos. 4,665,124; 4,940,646; 5,169,898; 5,169,897; 5,700,619 and 6,808,858. A photosensitive material containing a resin binder, including a vinylic or ethylenic polymer having phenolic hydroxyl groups and aromatic diazonium salt having only a single diazo group was disclosed in U.S. Pat. No. 4,374,193. and (cf. for example H. Baumann and H.-J. Timpe: “Chemical Aspects of Offset Printing” in J. prakt. Chem. Chemiker-Zeitung 336 (1994), pages 377-389). In all the above cases, the acetal containing resins are used in combination with diazonium salt compositions and only in conventional analog imaging and printing applications, as opposed to digital applications.
More recent developments in the field of lithographic printing form precursors have provided radiation-sensitive compositions useful for the preparation of direct laser addressable printing form precursors. Digital imaging information can be used to image the printing form precursor without the need to utilize an imaging master such as a photographic transparency, template or mask.
Today, many imaging processes use infra-red or near infra-red radiation from semiconductor diode lasers to image printing plates. Semiconductor diode lasers have the advantage of being much less expensive than ultraviolet lasers of the same power. They are also well adapted for the production of high resolution images and for digital imaging processes (i.e., for producing hard copies of images stored on computers in digital form). The cost per exposure unit intensity is less for an infrared producing high-resolution addressable source than for a comparable ultraviolet radiation producing source. The above fore-mentioned patents utilizing diazonium salt compositions do not have good natural sensitivity to near infra-red or infra-red laser irradiation.
U.S. Pat. No. 4,708,925 describes a negative working plate system with additional process steps performed after imaging and pre-development. In this disclosed process, the decomposition by-products are subsequently used to catalyze a cross-linking reaction between resins that insolubilize in the imaged areas prior to provide the development step in the formation of the printing plate. However, the process of this patent requires two exposure steps to be utilized as a negative-working plate, i.e., an image-wise exposure and a subsequent overall exposure.
U.S. Pat. No. 5,705,322 has a similar two step exposure process with a diazonaphthoquinone containing composition, except in this case the image-wise exposure is carried out with a near infra-red laser. The additional process step requirements add greatly to the cost and complexity of the process. The resins described in both patents are novolaks of phenol and formaldehyde.
U.S. Pat. Nos. 5,340,699; 5,372,907; 5,372,915; and 5,663,037 describe near infra-red laser imageable negative working systems that utilize a latent Bronsted acid or triazine with an infrared light absorbing dye. The resin system that is used is a combination of novolak and resol resins. U.S. Pat. Nos. 5,763,134, 6,605,416 and RE38,251 describe a negative working infra-red laser imageable composition in which specific squarylium dyes are used. The binder system that is described is again a novolak polymer.
The above described printing form precursors of the prior art which can be employed as direct imaged negative working printing form precursors are lacking in one or more desirable features. Moreover, these printing form precursor systems have constraints on their components which create difficulties in optimizing plate properties to provide optimum performance across the wide range of demanding lithographic plate performance parameters, including pre-heat temperature, developer solubility, solvent resistance, runlength and adhesion.
U.S. Pat. No. 6,541,181 describes acetal binder materials for use in a positive working system utilizing thermal lasers. In this format of process, the exposed areas are washed away during the development process rather than become the ink-retaining image to be printed onto the blanket or ink-receiving surface.
Photosensitive lithographic printing plates and those used for (Computer to Plate) CtP imaging is used in extremely demanding circumstances. Therefore, the coating compositions that are used for creating the image must meet exacting performance standards.
The properties of the photosensitive composition can be improved by several commercial and literature-disclosed methods. One method is to find new ingredients such as initiators, colorants, crosslinking agents, additives, etc. Another method is to optimize the amounts of each ingredient. Still another method is to optimize the conditions and equipment used to expose and develop the lithographic printing plates. The approach used in this disclosure to improve the plate coating composition characteristics and hence the plate performance is to provide novel polymer systems that enhance the physical properties and the reaction performance properties of the coating. This approach is likely to afford greater benefits than the other approaches because the polymer system represents the largest single element of any plate coating and the composition performance tends to be the limiting parameter with respect to image quality. Particularly, this approach is of decisive importance for lithographic printing plates since the behavior in the development of the image, both in exposure and development and the printing process, such as, for example, ink acceptance, scratch resistance and press life, is critically influenced by the polymeric binders.